Forming presses are widely used to form metals and other materials into shaped products. Typically, the material to be formed is placed in a lower die resting on the bed of the forming press and formed into the finished product by forces exerted by an upper die mounted on a movable and massive slide reciprocating in the press.
Air cushions are used in forming presses to provide blank holding for drawing metals or other materials and as ejectors to raise parts out of the lower dies. The air cushion is depressed by the moving top die, which contacts the air cushion through cushion pins directly or with the work piece therebetween. The stroke length of the air cushion is rarely in excess of one half the stroke of the slide holding the top die. Therefore, the air cushion does not begin to be depressed until a given point of the down stroke of the press slide is reached. Similarly, the air cushion returns to its top stroke position at some point on the return stroke of the press slide.
The air cushion would be subject to a severe impact upon its return to its top stroke position, therefore systems have been developed for snubbing the motion of the air cushion returning to its top stroke position. It is desirable to decelerate the moving air cushion as quickly as possible with the minimum amount of impact. These prior snubbing systems have compressed air at atmospheric pressure within a chamber fitted with a small orifice to form the snubbing action. With air at atmospheric pressure, a substantial compression ratio is necessary within the snubbing chamber to equalize the cushion force developed within the air cushion. In order to accomplish this task, the chamber has been fitted with a small orifice to prevent too much air escapement while air is being compressed within the snubbing chamber. It is therefore apparent that the time necessary for the last increment of air cushion travel near the top stroke position on the return stroke varies over a very broad range when the air cushion pressure and cushion return velocity varies.
Therefore, a need has arisen for a snubbing system which provides snubbing of the air cushion as it returns to its top stroke position which provides deceleration within a controlled time period with minimal variation due to the change in cushion pressure and cushion return velocity.